1. Field of the Invention
The present invention relates to a CAD (Computer Aided Design) system for designing a three-dimensional shape.
2. Description of the Related Art
In recent years, a CAD system for using a computer to design industrial products and buildings has been widely used in order to shorten the time for designing and improve the design quality. In many three-dimensional CAD systems, there have been employed a method of processing a model in units of shape feature, such as protrusion, holes and the like. Also, many of the three-dimensional CAD system include “a feature parametric function” capable of changing a shape easily by changing parameters that define the feature. In many three-dimensional CAD systems equipped with the feature parametric function, the feature constitutes a unit formed in a single drawing and also a unit to be subjected to edition such as subsequent deletion or change of order in creating history.
In the three-dimensional CAD system having the feature parametric function, the shape of a tap hole (stepped hole), for example, can be created by any one of following three methods. Namely, in a first method, a hole having a depth “a” is formed, and also a hole having a depth “b” from the bottom surface of the hole having the depth “a” is formed, to create the shape of the tap hole. In a second method, one half of the shape of the longitudinal section of a tap hole is rotated by 360 degrees to create the shape of the tap hole. A third method uses exclusive commands for creating a tap hole.
However, according to these methods, there has been caused the following problems. Namely, in the first method, the tap hole is created with two holes, and therefore configured of two features. Thus, subsequent editing operation is not easy. In the second method, a longitudinal sectional view of the tap hole is required. Therefore, in the case where only a plan view taken from just above the tap hole is available, a longitudinal sectional view is required to be prepared anew. The third method lacks flexibility, since the exclusive commands can create only a predetermined shape. Thus, the working efficiency of creating a three-dimensional shape is not so high.
In view of these conventional problems, an object of the present invention is to provide a technique for creating a three-dimensional shape, in which one feature is configured based on a plurality of loops with the contour lines thereof not crossed with one another, while at the same time making it possible to input an arbitrary extrusion size for each of the loops, thereby improving the working efficiency for creating the three-dimensional shape.